Transient optical power suppressing apparatus, method, and network

ABSTRACT

An apparatus for suppressing optical power transients includes a variable optical attenuator receiving an input optical signal and outputting an output optical signal; an optical power sensing element coupled to the input optical signal and sensing a portion of the input optical signal; and a feedforward loop controller coupled to the variable optical attenuator and to the optical power sensing element; the feedforward control loop providing feedforward control of the variable optical attenuator to reduce optical power transients of the input optical signal and maintain a substantially constant output power based on the input optical power and a reference value; the variable optical attenuator having a default opaque state in which the input optical signal is substantially attenuated when power is not being supplied to said variable optical attenuator. Variations include feedback loop controllers and a combination feedback and feedforward loop controllers.

This application is a divisional of U.S. patent application Ser. No.10/855,385, filed on May 28, 2004 now U.S. Pat. No. 7,483,205, whichclaims priority on Provisional Patent Application Nos. 60/500,241, filedon Sep. 5, 2003 and 60/473,917, filed on May 29, 2003, the entirecontents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention generally relates to methods and apparatuses forcontrolling or otherwise suppressing transients and their applicationsto various optical communication networks.

2. Description of Related Art

Optical WDM (wavelength division multiplexed) and other types of opticaltransmission systems are becoming increasingly dynamic. For example,there is a greater emphasis on dynamic add/drop architectures,protection switching, and traffic rerouting all of which can introduceunwanted optical power transients. In such systems, optical powertransients may also be introduced by various other factors such asroutine system activities and unintended events such as fiber cuts andequipment failures. Once an optical power transient is created it isoften exacerbated by optical amplifiers and other active opticaldevices.

The performance of the optical transmission system can be adverselyaffected particularly by fast power transients, beyond the effects ofthe temporary change in the power level at the receiver. Both themagnitude and temporal characteristics of the optical power transientsthat occur can limit the performance of various elements such as opticalreceivers and amplifiers in the system. While component designerscontinue to improve the transient performance of individual sources, acomprehensive strategy for suppressing the power transients that existin these systems offers improved system design flexibility andcoincident cost savings.

Rapid changes in optical power levels occur routinely in transmissionsystems. Reconfigurable optical add/drop architectures are designed toaccommodate changing levels in the total optical power present in agiven fiber or at the input to various optical components but theseaccommodations are often insufficient.

Other sources of optical power transients include optical protectionswitching mechanisms that can lead to changes in the optical power levelon the order of the speed of opto-mechanical switches, typically 100 to200 μsec. Fiber cut events can be faster, reaching a few 10ths of μs.System upgrades and maintenance often require changes in power levelsthat give rise to optical power transients.

Even a small optical power change from any of these sources can beexacerbated by constant gain amplifiers such as constant gainerbium-doped fiber amplifiers (EDFAs) particularly if the time scale issimilar to the time response of the EDFA control loop. The effects wouldbe even worse for optical amplifiers operated in constant power mode.The ultimate power transient after a cascade of EDFAs can beparticularly problematic. FIG. 15 shows one example of a conventionaloptical network configuration where one WDM channel is added at Node 1,and a second WDM channel is added at Node 2. If the fiber between Nodes1 and 2 is cut, EDFA #3 will experience a rapid 3 dB change (insituations where both signals have the same optical power) in its inputpower level, and the actual gain of EDFA #3 may experience a briefexcursion from its target value due to the finite response time of itscontrol loop.

Similarly, equipment failure or active optical components can introducesudden changes in the input power at an optical amplifier. An example ofthe effect of a simulated fiber cut (optical transition time 200 μsec)at the beginning of a cascade of 10 EDFAs is shown in FIG. 16. In thiscase there are two optical channels present at the input of the cascade(EDFA #2 in FIG. 1), and one of these is switched off to simulate afiber cut. The change in input power at the first EDFA is 3 dB, and ifthe EDFA control loop is capable of maintaining constant gain the secondchannel would be unaffected. However, the small transient (0.5 dB)observed at the output of the first EDFA induces a much larger transient(5 dB) at the output of the tenth amplifier. The performance of theEDFAs is limited by both the fundamental Erbium dynamics and the designand implementation of the EDFA control loop, which has a finite responsetime. In addition to the temporary power transient, the differencebetween the initial and final steady-state power levels increasesthroughout the cascade, due to the input power dependence of the gainaccuracy of each EDFA.

The above are non-limiting examples of the situations in which opticalpower transients arise and how they may be exacerbated. It is to beunderstood that many other examples and situations exist.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a high level block diagram of a transient optical powersuppressor according to a first embodiment of the invention andincluding both a feedforward control loop and feedback control loop;

FIG. 2 is a high level block diagram of another transient optical powersuppressor according to a second embodiment of the invention andincluding a feedback control loop;

FIG. 3 is a high level block diagram of yet another transient opticalpower suppressor according to a third embodiment of the invention andincluding a feedforward control loop;

FIG. 4 a is a block diagram illustrating a construction of a loopcontroller according to the invention and usable with the firstembodiment of the invention;

FIG. 4 b is a block diagram illustrating a construction of another loopcontroller according to the invention and usable with the secondembodiment of the invention;

FIG. 4 c is a block diagram illustrating a construction of yet anotherloop controller according to the invention and usable with the thirdembodiment of the invention;

FIG. 5 is a block diagram of an alternative loop controller utilizinglogarithmic amplifiers according to the invention and illustratingprinciples usable with all the embodiments of the invention;

FIG. 6 a is a diagram of an analog circuit that may be utilized toconstruct the loop filter according to the first embodiment of theinvention;

FIG. 6 b is a diagram of an analog circuit that may be utilized toconstruct the loop filter according to the second embodiment of theinvention;

FIG. 6 c is a diagram of an analog circuit that may be utilized toconstruct the loop filter according to the third embodiment of theinvention;

FIG. 6 d is a diagram of a digital circuit that may be utilized toconstruct the loop filter according to any of the embodiments of theinvention;

FIG. 7 a is a high-level block diagram illustrating the application ofthe inventive transient optical power suppressor towards protecting andenhancing the functionality of a receiver network element;

FIG. 7 b is a high-level block diagram illustrating the application ofthe inventive optical power suppressor towards protecting and enhancingthe functionality of an amplifier network element;

FIG. 8 illustrates a conventional ring-shaped optical network topologyuseful for illustrating how optical power transients arise inmulti-wavelength networks;

FIG. 9 illustrates a convention optical network having a mesh topologyand further illustrating how optical power transients may arise in suchnetworks;

FIG. 10 illustrates the application of the inventive transient opticalpower suppressor to an optical network having two ring-shaped networksinterconnected by an add/drop multiplexer;

FIG. 11 illustrates the various applications of the transient opticalpower suppressor according to the invention to an otherwise conventionaloptical network and further illustrating various possible locations andapplications to such a network;

FIG. 12 is a signal diagram illustrating a preferred operation of theinvention for compensating both positive and negative optical powertransients and the advantageous application of mid-range biasing for thevariable optical attenuator;

FIG. 13 is a simulated frequency response for different input powersshowing the simulated performance of the invention and furtherillustrating the inventive advantages;

FIG. 14 is a graph of transient amplitude versus transient suppressionresults and further illustrating experimental results of the invention;

FIG. 15 is a high level simplified block diagram of a conventionaloptical network having several nodes and two channels and furtherillustrating how a fiber cut may lead to optical power transients; and

FIG. 16 is a graph illustrating the response of the conventional networkof FIG. 15 to transients, particularly when a cascade of amplifiers ispresent.

DETAILED DESCRIPTION OF INVENTION

The following detailed description of the invention refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. Also, the following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims and equivalents thereof.

The expression “optically communicates” as used herein refers to anyconnection, coupling, link or the like by which optical signals carriedby one optical system element are imparted to the “communicating”element. Such “optically communicating” devices are not necessarilydirectly connected to one another and may be separated by intermediateoptical components or devices. Likewise, the expressions “connection”and “operative connection” as used herein are relative terms and do notrequire a direct physical connection.

Optical power transients are described by two general features; therelative magnitude of the power excursion from the steady-state leveland the time scale (temporal characteristics) of the transition inoptical power. For gaussian pulses or step-function changes, it isappropriate to consider the rise-time and/or fall-time of the change inoptical power level. The evolution of a transient pulse in a cascade ofamplifiers such as EDFA's will be governed largely by the speed and theamplitude of the transient, although other factors including any changein spectral loading and initial EDFA operating conditions are oftenimportant.

One apparent impact of positive optical power spikes is the potentialfor damage to optical receivers and other components. The damagethreshold of most optical components has a peak-power (electronicdamage) component and an average power (thermal damage) component. Thesusceptibility to damage is therefore a function of both the speed andamplitude of positive power transients.

A second consideration is data integrity, which is limited by thesensitivity of the detection circuit to changes in average opticalpower. The effects of the power transients at the receiver also haveboth a speed component and an amplitude component. The amplitude isimportant for several reasons. The most apparent is the fact that thedetection circuitry is typically designed to operate within a particularrange of average optical power levels, and a power transient ofsufficient amplitude will result in a closure of the eye diagram anddegraded system performance. Often optical power levels are configuredto maximize OSNR (optical signal-to-noise ratio) at the receiver, andthe arrangement of optical amplifiers and other components is typicallyselected with this in mind, but a temporary change in the power level islikely to result in a degraded OSNR at some locations.

Some system designs are limited by nonlinear effects that result inrestrictions to the allowable launch power into a fiber, and positivepower transients will temporarily perturb these levels and result inworsened nonlinear effects. The speed of fluctuations in average powercan also lead to data errors. The receiver circuitry should be designedto respond to data rate frequencies while also allowing for slowerdrifts in average power. If the speed of an unintended optical powertransient falls in between these two limits, data errors may result.These and other factors argue that an overall reduction in themagnitudes and speeds of optical power transients can lead to improvedsystem performance.

FIG. 1 illustrates an apparatus 100 for suppressing optical powertransients according to an embodiment of the invention. This apparatus100 is also referred to as a transient optical power suppressor and FIG.1 illustrates a first embodiment thereof.

Essentially, the transient optical power suppressor 100 illustratedtherein has an input optical signal having an input optical power P_(i).The output of apparatus 100 is another optical signal whose transientoptical power fluctuations have been suppressed. The optical power ofthe output signal is labeled as P_(o) and may otherwise be referred toas the optical output power.

The input optical signal may be any optical signal and the invention hasbroad applicability to reducing optical power transients in a widevariety of optical signals. The invention does have particular andadvantageous application to dynamic optical transmission systems havinga plurality of optical channels. Such dynamic transmission systems are,in and of themselves, quite common and include such technologies aswavelength division multiplexing (WDM) and reconfigurable add/dropmultiplexing as well as other channel multiplexing techniques such aspolarization state, sub carrier multiplexing, code divisionmultiplexing, etc.

Furthermore, the invention is insensitive to data rate, data format,analog or digital signals, actual wavelength or frequency of the data,number of transmission channels present, channel spacing, and variousfactors as the various embodiments of the invention seek to control onlythe transient optical power fluctuations which are independent of suchfactors.

The transient optical power suppressor 100 shown in FIG. 1 includes twocontrol loops: a feedforward control loop including optical powersplitting element 5, optical power sensing element 10, loop controller70, and variable optical power attenuation device 30. The second controlloop is a feedback control loop which includes optical power splittingelement 55, optical power sensing element 60, loop controller 70, andoptical power attenuation device 30.

While it is possible to provide a separate loop controller and variableoptical power attenuation device for each of the control loops suchduplication of parts is unnecessary, especially for a digitalimplementation. Only one optical power attenuation device 30 ispreferably used between the feedforward and feedback control loops. Asingle variable optical power attenuation device 30 should be sufficientto control all expected power fluctuations and supplying a secondvariable optical power attenuation device may introduce unwanted systempenalties such as additional insertion loss.

On the other hand, there is a technical reason for using multiple VOAsthat could outweigh these factors: namely, the limited dynamic range ofone VOA such that a different VOA could be used for feedback than forfeedforward control. Also, one high speed VOA could be used togetherwith a slow speed VOA with one latching for the other. Two (or evenmore) VOAs may also be advantageous for expanded dynamic range.

In more detail, the transient optical power suppressor 100 shown in FIG.1 includes optical pathways indicated by large arrows and electricalpathways indicated by smaller, single line arrows. The optical pathwaysmay be provided via optical fiber links that may be optically spliced tothe other elements or connected via standard connectorization methods.Alternatively, the optical components could be integrated into one ormore substrates. It is also possible that all of the components shown inFIG. 1 could be integrated into a single substrate.

For example flip chip techniques have been used to put PIN or APDdevices on the same substrate as other optical devices such as couplers,taps as required here. VOA technologies required for suppression oftransients discussed here lend themselves to integration with passivetaps and couplers on the same substrate. The loop filter is likely to bea separate chip with current technology but perhaps could be integratedwith the other components in the future.

Devices have been constructed integrating all of the optical componentsemployed by the invention (fast VOAs, optical power taps and PINs) ontoa single Si chip already. All of the chips used in the control loop arevery often fabricated on Si, so it is straightforward to integrate thesetogether with the optical components on one chip. However, such animplementation would be less attractive if the expected volume is low orif compactness and ease of manufacture are not critical issues, at leastwith current manufacturing techniques in this area. VOA technologiesrequired for suppression of transients discussed here led themselves tointegration with passive taps and couplers on the same substrate.

As further shown in FIG. 1, the optical input signal whose power isrepresented by P_(i) is split by an optical power splitting element 5which splits the optical signal such that a major portion thereof issupplied to the variable optical power attenuation device 30 and asmaller portion thereof is supplied to the optical power sensing element10. Likewise, the optical power splitting element 55 splits the opticalsignal output from the optical power attenuation device 30 and suppliesa major portion thereof to the output port having output power P₀ and amuch smaller portion thereof which is supplied to the optical powersensing element 60. The splitting or tap ratio of the optical powersplitting elements 5, 55 is generally selected to supply powersufficient for the sensing elements 10, 60 while preserving most of thepower for the main signal pathway. Such optical power splitting elements5, 55 are very conventional and may be constructed utilizing variousdevices such as couplers or taps.

The optical power sensing elements 10, 60 may be constructed withconventional PIN photodetectors, avalanche photodetectors (APDs) orequivalent elements. Such power sensing elements are highly conventionalin and of themselves.

In the feedforward control loop, the optical power sensing element 10senses a portion of the input optical signal and outputs a signalindicative of the power P_(i) of the input optical signal. This signalIN1 is indicative of the input optical power and is supplied to the loopcontroller 70 as shown. In the feedback control loop, the optical powersensing element 60 senses a portion of the output optical signal inorder to measure power of that signal and outputs an electric signal IN2indicative of the power of the output optical signal. This electricalsignal IN2 is supplied to the loop controller 70 as shown.

The loop controller 70 is a shared controller that controls both thefeedforward and feedback control loop operations. The loop controller 70may be constructed with a variety of devices an example of which isshown in FIG. 4 a and another example of which is shown in FIG. 5.Further details of the loop controller 70 will be explained withreference to those figures below.

The loop controller 70 has electrical connections to the optical powersensing elements 10, 60 and accepts the optical power measurementsignals IN1, IN2 representative of the optical power of the input andoutput optical signals, respectively. The loop controller 70 may alsoaccept as input a reference value (REF) which may simply be a voltagevalue externally supplied or internally stored within the loopcontroller 70 itself. The reference value provides a basis against whichthe loop controller may control the variable optical power attenuationdevice 30 so as to output a substantially constant power optical signaloutput P₀. Further details of the operation of the loop controller 70are provided below. The control signal provided by loop controller 70 islabeled as OUT and is supplied to the variable optical attenuationdevice (VOA) 30.

The variable optical power attenuation device 30 may be constructed froma variety of devices such as a Mach Zehnder (MZ) optical modulator, orvarious other devices. Since the speed of this device 30 is a criticalcomponent of the invention, technologies such as the current-injectionVOA are generally preferred because of their fast response time. As thetechnology develops and other competing VOA technologies develop thathave response times on the order of the current-injection VOAtechnology, then such future technologies would be preferred assumingthat costs and reliability and other factors are relatively equal.

Because the overall inventive aim is to suppress optical powertransients and because such transients have a time scale that can beextremely short, it is important to utilize a variable optical powerattenuation device 30 that has a fast response time. The preferredtechnology chosen is a current-injection variable optical attenuator(current injection VOA). Attenuation of this type of device is roughlyproportional to drive current which is shown as the OUT or controlsignal from the loop controller 70.

The frequency responses of the current-injection VOA and its drivecircuit can be described as

$\frac{P_{out}}{I_{VOA}}(f)\mspace{14mu}{and}\mspace{14mu}\frac{I_{VOA}}{V_{in}}(f)$respectively, where P_(out) is the optical power measured at the VOAoutput (P₀ in FIG. 1), I_(VOA) is the drive current (OUT in FIG. 1), andV_(in) is the reference voltage at the comparator (REF in FIG. 1).

The current-Injection VOA technology provides the fastest response timecurrently available from commercial VOA devices, and response time iscritical for this invention. The device is typically based on a Sisubstrate, in order to leverage the vast amount of technologicalexpertise and processing equipment that are based on Si thin filmdevices. The device includes one or more optical waveguides, which canbe fabricated from Si or SiO2 layers that are deposited onto thesubstrate and then patterned into channel waveguides. A cathode andanode are situated above and below a segment of the waveguide, andelectrical current is passed through either the guide region itself orthe cladding region adjacent to the guide. The materials are selectedsuch that the electrons or holes injected are efficiently absorbed byphotons propagating in the guide (and partially in the cladding modes aswell) and the light is thus absorbed and converted into thermal energy.The degree of attenuation will vary with the amount of electricalcurrent injected. The process is extremely fast, limited mainly by theelectronic properties of the current source and the impedance propertiesof the materials involved. Typically response times of less than onemicrosecond are achievable using this technology.

Returning to FIG. 1, the variable optical power attenuation device 30 isthe main active element that acts upon the input optical signal in orderto suppress or otherwise reduce transient optical power fluctuationsunder the control of the loop controller 70. The transient-reducedoptical signal having output power P₀ is output from the apparatus 100.Operational details are supplied below.

FIG. 2 illustrates another embodiment of the invention and isessentially the feedback control loop transient optical power suppressor200 as shown. Except for the loop feedback controller 270, all of theelements shown therein have been fully explained above with reference toFIG. 1. Further details of the loop controller 270 and its operation areexplained below with reference to FIG. 4 b. It is important to realizethat the feedback control loop embodiment may operate independently anddoes not necessarily require the feedforward control loop additionalelements shown in FIG. 1. In other words, the feedback control looptransient optical power suppressor 200 represents a distinct embodimentof the invention.

FIG. 3 is the feedforward control loop 300 embodiment of the inventionand also acts as a transient optical power suppressor. Again, most ofthe elements shown in FIG. 3 have also been described above withreference to FIG. 1. Like reference numerals indicate like elements anda repeated description thereof is unnecessary here. The main differenceis the feedforward loop controller 370 which does differ from the loopcontroller 70 of apparatus 100 shown in FIG. 1. Further details of theloop controller 370 and its operation will be explained below withreference to FIG. 4 c.

FIG. 4 a illustrates the components of loop controller 70 that may beutilized with the transient optical power suppressor 100 shown inFIG. 1. The loop controller 70, in general, utilizes the signals IN1,IN2 from the optical power sensing elements 10, 60 to derive a drivesignal (out) for the variable optical power attenuation device 30. Inmore detail, the loop controller 70 includes amplifiers 70, 78 each ofwhich may be a unity-gain buffer (gain=1) or may have multipleamplification stages with more or less complex frequency responses andgains such as the logarithmic amplifiers discussed in relation to FIG. 5below. Furthermore, the amplifiers 72, 78 may be completely omitted fromthe loop controller 70. These amplifiers 72, 78 may be constructed withsimple IC circuits typically referred to as operational amplifiers andare available form various manufacturers such as Texas Instruments orothers.

The reference value (REF) may be a voltage or current signal that may beproportional to the desired optical output power level or opticalattenuation. Generally speaking, optical attenuation is the ratiobetween P₀ and P_(i) and can be smaller than one if the optical powerattenuation device 30 can amplify the optical signal present at itsinput port.

It is also possible that the REF signal may be adjusted. Such adjustmentmay be accomplished via a simple potentiometer or may be supplied by anexternal device such as node controller, network controller, systemsadministrator, craft terminal, etc. If the REF signal is connected to aservice channel or overlay network, it would also be possible toremotely designate or change the attenuation value to provide moredynamic control of the various embodiments of the invention.

The REF input to the loop controller 70 can take different formsdepending on the particular implementation of the loop controller. Ingeneral, the range and scaling of REF will be similar to the range andscaling of the quantities that derived from the optical power signalsand used by the loop controller 70.

For example, in an implementation where the optical power signals areconverted to voltages and the loop controller 70 is an analog electroniccircuit, REF could be a simple voltage generated for example with apotentiometer.

Another example would be a case where the controller is implementedusing digital techniques (e.g. microprocessor). The optical powersignals could be converted to digital numbers and REF would then becomea simple number that the digitized power signals could be compared to.

The loop filter/processing element 75 and/or the loop controller 70itself may be constructed utilizing various components such as analogcircuits, digital circuits, or a combination of both. The overallpurpose of the loop filter/processing element 75 is to improve thedynamic performance of the power/attenuation control loops (e.g.,transient suppression efficiency and speed).

Possible examples of loop filter/processing element 75 can range fromsimple analog R-C filters built around operational amplifiers, leadingto proportional integrators or derivators (known as PI or PIDcontrollers) such as that shown in FIG. 6 a-c, to more complexmicroprocessor or digital signal architectures as generally known in theart.

FIG. 6 d shows one non-limiting example of a digital implementation ofthe loop filter/processing element 75 that also applies to the otherloop filter/processing elements 175, 275 of the second and thirdembodiments.

As shown in FIG. 6 d, the REF signal may be provided to a microprocessoror a microcontroller 470 via the control software 480, and could be, forexample, an external user-supplied value or a value from a look-uptable. This easily permits the REF signal to be programmable.

The A/D converter(s) 460 supply digital versions of the analog powermeasurements IN1 and/or IN2 (depending upon which embodiment, differentpower values are input) and may be omitted if the power values areprovided in a digital format.

The microprocessor/microcontroller 470 uses the digital values from theA/D converters 460 as input variables to the specific “filter equation”being implemented within the microprocessor/microcontroller programmedwith the control software 480. In the digital world, this “filterequation” is the mathematical equivalent of the circuits illustrated in,for example, FIG. 6 a, 6 b, or 6 c.

The D/A converter 490 takes the digital output from themicroprocessor/microcontroller 470 and supplies it to the output driver77; this element may also be omitted if the particular output driver mayaccept a digital input.

As an alternative to a microprocessor/microcontroller 470 and controlsoftware 480, an FPGA or some other ASIC could also be used to implementthe loop filter equation and otherwise replace themicroprocessor/microcontroller 470, as is well known in the art.

The design intent for which the loop filter/processor element 75 ismainly responsible is to suppress power transients utilizing anappropriate control signal that is supplied to the variable opticalpower attenuation device 30 in order to maintain a constant power levelor at least substantially constant power level at the output. This maybe done, in general, by finding the difference between the measuredpower and the reference value. This difference may be amplified andconditioned by the loop filter/processing element 75 in order to improvethe control loop performance. In this way, the loop filter/processingelement 75 supplies a control signal to the output driver 77.

The output driver 77 is responsible for converting the control signalfrom the loop filter/processing element 75 into an appropriate signalrequired by the particular variable optical power attenuation device 30chosen for the implementation. In other words, the drive signal willvary depending on the particular technology, manufacturer, and otherparameters of the particular variable optical attenuation device 30chosen for the implementation.

The output driver 77 is used to transition from the control signalsgenerated by the loop controller 75 to the drive signal required by thespecific VOA 30 being used. For example, in a case where the VOA 30 is avoltage- or current-controlled device and the loop controller 75 isimplemented as an analog electronic circuit, the output driver 77 couldbe a simple operational amplifier circuit to provide either the requiredvoltage gain or transconductance gain.

In a case where the loop controller 70 is implemented using digitaltechniques (microprocessors, DSP, etc. . . . ) such as shown in FIG. 6d, the output driver 77 could be a D/A converter or a combination of aD/A converter and an amplifier. Such driving circuitry and itsalternatives are conventional elements in and of themselves. FIG. 6 aillustrates one construction of the loop filter/processing element 75for the combination feedforward and feedback control loop embodiment ofthe invention. Further details of the operation thereof will beexplained below with reference to FIG. 6 a.

FIG. 4 b illustrates the loop controller 270 that may be utilized by thefeedback control loop embodiment of FIG. 2. The loop controller 270shares many components with the loop controller 70 as indicated by likereference numerals. Thus, a duplicative description will be omittedhere. The main difference with respect to FIG. 1 is the loopfilter/processor element 275. An example of a filter processor element275 is shown in FIG. 6 b with that figure representing but one exampleof how the circuit may be constructed utilizing analog components.Further details of the operation thereof will be provided below.

FIG. 4 c illustrates the feedforward loop controller 370 that may beused with the feedforward control loop embodiment of FIG. 3. Again, manyelements are shared with the loop controller 70 of FIG. 4 a. The maindifference is the loop filter/processor element 375 a construction ofwhich is explained below with reference to FIG. 6 c.

FIG. 5 illustrates an alternative loop controller 170 that may besubstituted for the loop controller 70 of the combined feedforward andfeedback control loop embodiment of FIG. 1. The loop controller 170mainly differs in the use of logarithmic amplifiers 172, 178 in place ofthe amplifier 72, 78 of FIG. 4 a. The other main difference is the loopfilter/processor element 175 which differs slightly from the loopfilter/processing element 75 of FIG. 4 a. The loop filter 175 willimplement a different transfer function when log amps are used due tothe differing gain range and output parameters as compared with linearamplifiers.

The use of a log amp in the loop controller is an important component tothe preferred implementation and is preferably used in all three mainembodiments. The function of the logamp is to convert an electricalinput into an amplified output that varies logarithmically with theinput. This allows a very large dynamic range of signal to be amplified,and avoids the requirement for switching the amplifier gain betweendifferent values (which causes a number of performance limitations).

The key performance requirements of the logamp for this application arespeed, dynamic range, and accuracy. Recently several logamps have comeonto the commercial market having significantly improved performance inthese areas, particularly speed. Specific non-limiting examples for thelogamps 172, 178 include:

Texas Instruments Logamp part number LOG114

Maxim logamp part number MAX4207

Burr-Brown part number OPA380

As explained below, the logarithmic amplifiers 172, 178 are generallypreferred over the amplifier 72, 78 of the FIG. 4 a embodiment. Thereasoning is discussed below and also applies to the amp 78 used in thefeedback loop controller 270 and the amp 72 used in the feedforward loopcontroller 370.

When modeling control loop for a VOA, an equation similar to thefollowing one can often be used to approximate the relationship betweenthe optical input power (P_(i)), the optical output power (P_(o)), andthe VOA control signal (x):

10 ⋅ log (P_(i)/P_(o)) = K ⋅ xK, although most of the time a complex nonlinear transfer function, canbe approximated by a constant in the vicinity of a specific operatingpoint.Assuming that P_(i) is constant, the small-signal gain between thecontrol signal x and the output power P_(o) can be determined by takingthe difference between the following equation and the following one:

10 ⋅ log (P_(i)/(P_(o) + Δ P_(o))) = K ⋅ (x + Δ x) Replacing the logarithm by its linear approximation around P_(o), onethen gets

${\frac{\Delta\; P_{o}}{\Delta\; x} = {{- K^{\prime}} \cdot P_{o}}},$where K′ is equal to K·ln 10/10.

This last equation shows that in the vicinity of a control point P_(o),the “gain” of the VOA 30 is dependent on the optical power level. In aclosed-loop system, this means that the dynamics of the VOA control loopwill be dependent on the specific operating point selected for the VOA,usually causing the loop to become very slow at low power levels, andpresenting the risk of oscillation due to increased gain at higher powerlevels.

A logarithmic amplifier delivers an output signal, usually a voltage,that varies as the logarithm of the ratio between the input signal tothe amplifier, usually a current, and some reference signal, alsousually a current. Designating as V_(o) the output of the logarithmicamplifier, I_(i) the input signal, and I_(r) the reference signal, thisinput-output relationship can be modeled as

V_(o) = K_(a) ⋅ log (I_(i)/I_(r)),where K_(a) is a constant and represents the gain of the amplifier.

Using a similar approach to the one used above for the VOA 30, thesmall-signal relationship between the amplifier input and output signalscan be written as

$\frac{\Delta\; V_{o}}{\Delta\; I_{i}} = {\frac{K_{a}^{\prime}}{I_{i}}.}$

Thus, the small-signal gain of the logarithmic amplifiers 72, 78 arounda certain operating point I_(i) is inversely proportional to the inputcurrent I_(i). By making I_(i) proportional to the optical power P_(o).For example by using photodetector 60, one can eliminate the dependenceof the loop gain on the optical power since the dependence in P_(o) andI_(i) in the equations above will cancel each other.

FIG. 6 a shows the more complex case where both feedforward and feedbackcontrol loops are used. Instead of using specific filter examples, FIG.6 a expresses the filters in terms their Laplace transformnomenclatures. Typically G3(s) is equal to 1/G2(s) although in practiceit is difficult to achieve specially for some frequency ranges. Thefeedforward filter loop will represent, therefore, some kind ofderivative function, if proportional integral (PI) is used for thefeedback control loop, the overall control loop can take the form of aPID controller. The precise definition of these transfer functions willgreatly depend on the VOA technologies used and the parameters tooptimize as is known in the art.

Note that the 2 reference levels for the combined feedforward/feedbackcontrol loop can be defined differently as REF1 refers to the opticalpower at the input of the device, while REF2 refers to power at theoutput of the device. The errors detected from both control loops areused to generate the proper drive signal to the VOA through G1(s), G2(s)and G3(s). A1 through A3 are differential amplifiers (and possibly gainelements) that provide at their output a possibly amplified signal thatcorresponds to the delta of their input. Amp2 is an amplifier, possiblya log amp that is used to amplify the signal coming from the sensor.Such an amplifier can also be used for IN1 but is optional.

FIG. 6 b illustrates one alternative for constructing the loopfilter/processing element 275 which applies in the feedback controlloop. Essentially, the loop filter/processing element 275 may beconstructed with a simple analog circuit as shown. The circuit includesa differential amplifier stage A1 which is utilized to compare theactual output optical power level (via IN2) to a desired output powerlevel (via REF value) and a proportional-integral (PI) filter stage A2.The proportional-integral filter stage A2 includes an associated RCcircuit including resistors R1, R2 and capacitor C connected as shown inFIG. 6 b. The values of the components R1, R2, and C are selected andtuned to optimize the control loop performance and will depend uponvarious factors such as the specific VOA 30 selected for theimplementation. The amp (Amp2) shown in FIG. 6 b corresponds to eitherthe logamp or other opamp implementations described above.

FIG. 6 c illustrates one alternative for constructing the loopfilter/processing element 375 which applies in the feedforward controlloop. Essentially, the loop filter/processing element 375 may beconstructed with a simple analog circuit as shown. The circuit includesa differential amplifier stage A1 which is utilized to compare theactual input optical power level (via IN1) to a desired output powerlevel (via REF value) and a proportional-integral (PI) filter stage A2.The proportional-integral filter stage A2 includes an associated RCcircuit including resistors R1, R2 and capacitor C connected as shown inFIG. 6 b. The values of the components R1, R2, and C are selected andtuned to optimize the control loop performance and will depend uponvarious factors such as the specific VOA 30 selected for theimplementation. The amp (Amp1) shown in FIG. 6 c corresponds to eitherthe logamp or other opamp implementations described above.

The simple analog filter illustrated in FIG. 6 c here could be used in asituation where only feedforward control is desired. In that case theinput power Pin is read and the VOA attenuation controlled such as tomake sure that Pin−VOA_attenuation matches the target output power. Sothe output power is not controlled directly but derived from the readingof the input power through the sensing element 10 and converted to acurrent or voltage IN1. Ref in that case corresponds to the voltagerequired to maintain a specific VOA attenuation VOA_attenuation suchthat Pin−VOA_attenuation corresponds to the desired output power.

The output from the differential amplifier is fed to theproportional-integral (PI) filter stage. The values of the componentsR1, R2, and C are selected and tuned to optimize the control loopperformance and will depend mainly on the specific VOA selected for theapplication.

A feedforward implementation is suitable for example when the outputpower would be too small after the VOA attenuation to get sufficientaccuracy in the control, as it would be the case in a feedback controlmechanism.

It is noted that the various embodiments of the invention are notlimited to the specific control themes specifically described above orillustrated in the drawings. Each of the control loops (feedback,feedforward and the combination of feedback and feedforward) may use avariety of control schemes such as proportional (P),proportional-integral (PI) or proportional-integral-derivative (PID). Asis also understood in the art, feedforward control schemes generallyprefer PID or at least a control scheme including a derivativecomponent.

VOA Default Opaque State

It is generally preferred to set the default state of the variableoptical power attenuation device 30 to be an “opaque” state. In otherwords, in all of the above embodiments, the optical power attenuationdevice 30 or VOA should be manufactured or set such that it has amaximum attenuation (opaque) in the default or power-off state. Furtherdetails follow.

The default state of a VOA device is typically defined to be eithertransparent (maximum transmission) or opaque (maximum attenuation). Thedefault state will occur if the VOA devise loses electrical power due toequipment failure or some other cause. Optical transmission systemarchitectures vary in structure, and different optimum default statesare preferable for different types of systems. Point-to-pointtransmission links may have little need for a particular default stateif the signals pass through optical amplifiers that will go dark anywayif power is lost at the node location, or the requirement may be defaulttransparent if the rest of the node is designed to pass through thetraffic if at all possible when power is lost.

Dynamic optical transmission systems having numerous added and droppedchannels benefit significantly if the VOA 30 becomes opaque if control(or power) is lost. One reason is to prevent duplication of a wavelengthchannel. VOA 30 may be used to squelch a particular wavelength that isreused downstream in the network. If the VOA 30 loses power for anyreason and becomes transparent, the unwanted signal that was intended tobe squelched could prevent the intended channel from being transmittedsuccessfully.

Another reason for a default opaque state for VOA 30 is fault isolationand signaling. The ability of a transmission network to respond tofailures of many types is a critical performance factor. If power islost at a node, or if a failure on a PCB (printed circuit board) causesa VOA or other device to revert to its default state, it is in manycases advantageous to squelch the unreliable signal and allowconventional LOS (loss of signal) detection equipment within the networkto either reroute traffic or to initiate signaling and alarms that willindicate the problem as quickly as possible.

Mid-Range Bias of VOA 30

The loop controller 70 and its alternatives 170 and 270 preferably biasthe variable optical power attenuation device 30 such that it is in itsmid-range or at least substantially close to a mid-range bias. This maybe accomplished via the loop filter/processing element 75 or itsalternatives 175 or 275. Alternatively, this can be accomplished via theoutput driver 77 applying appropriate DC bias levels to the VOA 30control signal. The rationale for this mid-range biasing and theadvantages achieved thereby are further explained below with referenceto FIG. 12.

Generally speaking, optical power transients may be designated aspositive transients that cause a sudden increase of the of the opticalpower level in a network or optical signal and as negative transientsthat cause a sudden decrease in the optical power. FIG. 12 illustratesthe positive and negative transient designation as well as theinvention's compensation for such positive and negative transients.

Although a VOA is often perceived as an element in an optical networkthat can only attenuate, or reduce, optical power it can be used to bothincrease or decrease the power level in an optical network if itssteady-state operating point (attenuation level) is somewhere in themiddle of its dynamic range. The VOA driver circuit 77, for example, canthen increase or decrease the attenuation to compensate for eitherpositive or negative power transients under the control of the loopcontroller such as controllers 70, 170 and 270. The compensation rangein either direction will depend on the specific steady-state operatingpoint selected and also on the total attenuation range of the VOA. FIG.12 illustrates the concept of VOA mid-range biasing.

Even if steady-state operation of the VOA 30 above its minimumattenuation level may be perceived as causing extra loss in a network,it is very often possible and even desirable to incur the extra loss andadjust optical power to lower levels in order to optimize performance ofthe network. A good example of this is in reducing the optical power ona receiver in order to prevent saturation and improve tolerance to powervariations.

The circuits of FIGS. 6 a-d is illustrative only and are not meant tolimit the invention in any fashion. Indeed, various other analogcircuits could be utilized to include proportional control, PID(proportional integral derivative control) or various other standardcontrol schemes as are known in the art. Furthermore, the invention isnot limited to analog implementation such as those shown in FIGS. 6 a-c.Indeed, the control may be represented as an equation and implementedvia software and/or hardware microprocessor circuitry as is also knownin the art.

Operation of Invention

Generally speaking, the various embodiments of the invention operate toreduce or otherwise suppress optical power transients in an opticalsignal. This may be accomplished utilizing the combined feedforward andfeedback control loop embodiment illustrated in FIG. 1, the feedbackcontrol embodiment 200 shown in FIG. 2 or the feedforward control ofembodiment 300 shown in FIG. 3. There are several variations for thecomponents of these embodiments as described above. In general, all ofthem generally operate by applying one or more control loops samplingthe input and/or output optical power values and supplying them to aloop controller. The loop controller controls the variable opticalattenuator 30 to reduce optical power transients and maintain asubstantially constant output power based on the power measurements. Thefeedforward control loop provides this control based on the inputoptical power measurement and a reference value. Likewise, the feedbackcontrol loop provides this control based on the output optical powermeasured by the optical output power sensing element and a referencevalue.

The operations of the feedforward and feedback may be combined toprovide a comprehensive control based on both input and output powermeasurements supplied by the optical power sensing elements 10, 60.Generally speaking, this control is effected by comparing a measuredpower against a reference value and this difference may be calculatedvia analog circuitry such as the differential amplifier shown in FIG. 6a or via microprocessor for software based implementations that applyequations. Additional levels of detail in the control equations,particularly when using a logarithmic amplifier such as the embodimentof loop controller 170 shown in FIG. 5 are referenced above. The resultof the control loop is a control signal supplied to the variable opticalpower attenuation device 30 such that it may respond to sensed opticalpower transients and thereby keep the output power substantiallyconstant.

Because optical power transients may be extremely fast in short livedevents, it is very important that the invention respond as quickly aspossible to suppress the transient and prevent it from being sent todownstream optical elements.

The speed of the control loop is limited by 3 separate factors. First,the VOA 30 speed is dependent on the technology chosen. VOA responsetimes vary from several seconds to sub-microsecond. In the latter case,the output driver 77 circuitry may be a limiting factor. Second, thebandwidth of the detection circuit including sensors 10, 60 used toprovide the feedback is important. Here the amplification scheme maylimit the overall frequency response. Third, the loop filter (75, 175,275) and differential amplifier should be chosen to maximize the overallresponse speed.

The dynamic range of the control loop is determined largely by the VOA30 dynamic range. For positive power transients, the dynamic rangeavailable below the operating setpoint will limit the amplitude oftransients that can be suppressed. In order to suppress negative opticaltransients, the VOA 30 setpoint must be adjusted to an attenuation thatis large enough to accommodate the negative amplitudes. In this case theVOA setpoint adds to the span loss in the transmission system or loss ofdemultiplexing elements after an optical amplifier and could limit theflexibility of system design. In the case the VOA is part of the line,like when the VOA is placed between 2 line amplifiers, the advantages ofsuppression of negative transients must be balanced against thedisadvantages of reduced link budgets.

Simulated and experimental data of the inventive operation was alsogathered thereby proving the advantages of the invention. Specifically,the feedback control loop embodiment of FIG. 2 was utilized for theanalysis. The optical power sensing element 60 was simulated as a PINdiode and the VOA 30 was constructed with a current-injection VOA type.For the amplifiers and the loop controller, logarithmic amplifiers wereutilized. For the loop controller 270 and specifically the loopfilter/processing element 275, the following factors were taken intoconsideration.

The frequency responses of the VOA 30 and its drive circuit 77 weremeasured independently and used to model the overall frequency responseof the control loop. This simulation also incorporated frequencyresponse data for the PIN, logarithmic amplifier, and comparator A1.Simulation results for the control loop are shown in FIG. 13. The figureincludes two families of curves: the solid lines represent the abilityof the control loop to follow variations to the reference voltage V_(r)and the dashed lines represent the ability of the Control Loop to rejectvariations of the input optical power. This second characteristicdetermines the overall effectiveness of the transient suppression, andin this implementation the frequency response was limited to roughly 10⁵Hz. The limitation is principally the result of the relatively poorfrequency response of the logarithmic amplifier used to convert theoutput PIN current to V_(o).

FIG. 14 shows the experimental results for a positive Gaussiantransient. As shown therein, the results are extremely promising and doshow and exhibit the inventive advantages of suppressing optical powertransients. The various curves represent different speed or timescalefor the input optical power transient.

APPLICATIONS OF THE INVENTIVE EMBODIMENTS

The various inventive embodiments may be applied in a wide variety ofsituations. Generally speaking, there are certain locations inconvention optical networks that would greatly benefit from theapplication of the invention. Two basic locations are illustrated inFIGS. 7 a and 7 b.

As shown in FIG. 7 a, the transient optical power suppressor 100 may beprovided just prior to a receiver node 500. Such receiver nodes ornetwork components are highly conventional elements in and of themselvesand typically include such components as a photodetector to detect theoptical signal and perhaps some timing and error correction circuitry.The actual implementation of receiver node 500 is irrelevant to theinvention. The point is that receiver nodes 500 may become damaged byoptical power transients. Optical power transients can also affect theperformance of network elements such as the receiver node 500 andproviding the invention at a network location prior to the receiver node500 has distinct advantages.

FIG. 7 b shows another typical application of the transient opticalpower suppressor 100 according to the invention. In this case, thetransient optical power suppressor 100 is provided just before anamplifier node 550. Such amplifier nodes 550 are conventional elementsin and of themselves and may include any wide variety of amplifyingcomponents. Most typically in the WDM network, the amplifier node is anEDFA (Erbium doped fiber amplifier) but of course a wide variety ofother optical amplification techniques and components may be utilized.

Other more detailed examples of network locations that would benefit byapplication and the invention are described below in relation to FIG.8-11.

FIG. 8 shows a ring-shaped optical network with 4 nodes, labeled N1 toN4. λ1 to λ4 represent a wavelength or a set of wavelengths used totransmit information between these nodes. A1 and A2 represent opticalamplifiers. λ2 is being transmitted from Node 1 (N1) to node 3 (N3). Indoing so it goes through 1 amplifiers, A1. Say the fiber between N1 andN2 is cut. In that case the traffic λ1 is being lost obviously, as wellas the traffic λ2. The traffic λ3 should not be affected, however as thenumber of wavelengths going the amplifiers A1 is being changed, thepower level of λ3 will be experiencing variations (transients). Thecause of these transients is that the amplifiers A1 can notinstantaneously find a new inversion level that will accommodate thereduced power levels at its input. This will be true regardless of theamplifier being used in power control mode, or gain control mode,although the impact will be different in these 2 cases. Because thispower fluctuation can cause error rates at the receivers, it isimportant to be able to correct these fluctuations with an opticaldevice, in this case a fast Variable Optical Attenuator (VOA) 30 andattendant control loop(s) as disclosed herein. In FIG. 11 there is showndifferent possible placements for the VOAs.

FIG. 9 shows a mesh network with 6 nodes, labeled 1 to 6. λ1 to λ6represent a wavelength or a set of wavelengths used to transmitinformation between these nodes. A1 and A2 represent optical amplifiers.As in FIG. 8, different traffic paths share some fiber segments. Say thefiber between N2 and N5 is being cut. The traffic λ3 and λ5 is beinglost. λ4 should not be affected, but because of the load change in theamplifiers A1 and A2, its optical power varies, requiring fast VOA forpower control to minimize performance impact.

FIG. 10 shows a ring (Ring 1) being connected to another ring (Ring 2).The traffic λ2 is originating from N4 in Ring 1 and being dropped at N3in Ring 2. To do this, it goes through N5 in Ring 1 and N1 in Ring 2.Note that N5 and N1 can be the same physical node. When multiple ringsare connected together it becomes very important to isolateperturbations from one ring to another. I.e. any event that would causepower fluctuations in ring 1 must not affect the traffic in Ring 2. Alsothe number of rings connected together is not limited to 2. At anynodes, another ring can be connected and share that node.

It is important, in that specific example, to use the transient opticalpower suppressor 100 in between the Demux and Mux of N5 (Ring 1) and N1(Ring 2). When multiple wavelengths are transmitted between the rings,and array of VOAs, the transient optical power suppressor 100, should beused, one per wavelength. The use of the transient optical powersuppressor 100 will prevent power fluctuations to travel from one ringto another, simplifying network management, planning and systemperformance calculations.

FIG. 11 aims at showing in a typical application where the transientoptical power suppressor can be used to control power fluctuations andtransients. Note that the link showed here could be part of any topologydescribed earlier, ring, mesh, ring-ring interconnect, etc. Variouspossible placements for the inventive transient optical power suppressor100 are shown therein and further described below:

-   -   1: After a transmitter. When a laser fails, or the power supply        in a specific shelf fails the light at the output of the laser        can suddenly go down or up. The VOA transient power suppressor        100 is used to stabilize that power and avoid fast fluctuations        to propagate down the link.    -   2: Before an amplifier. In that case one the transient power        suppressor 100 is used to control the total power at the input        of the amplifier. The goal is to limit fast power fluctuation at        the input of the amp. This will allow to minimize the        requirement of fast power or gain control loop inside the        amplifier itself.    -   3: After an amplifier. For the same reasons as in P2, but this        time placed after the amplifier.    -   4: Before a receiver. Finally the transient optical power        suppressor 100 before a receiver and is used to control a single        wavelength and maintain at the receiver as constant as possible        the optical power level. Maintaining a fixed power level at the        receiver is important for multiple reasons:        -   Surge up in power can cause bit errors and damage the            receivers.        -   Surge down in power can cause bit errors or even sync loss

Although the first embodiment 100 of the invention is shown in FIGS. 7a, 7 b, 10 and 11, it is to be understood that any of the inventiveembodiments may be placed at the location indicated in these figures.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded asdeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. An apparatus for suppressing optical power transients, comprising: avariable optical attenuator receiving an input optical signal andoutputting an output optical signal; an optical power sensing elementoptically coupled to the output optical signal, said optical powersensing element sensing a portion of the output optical signal andoutputting a signal indicative of output optical power of the outputoptical signal; and a feedback loop controller operatively coupled tosaid variable optical attenuator and to said optical power sensingelement; said feedback control loop providing feedback control of saidvariable optical attenuator to reduce optical power transients of theinput optical signal and maintain a substantially constant output powerbased on the signal indicative of the output optical power and areference value; said variable optical attenuator having a defaultopaque state in which the input optical signal is substantiallyattenuated when power is not being supplied to said variable opticalattenuator; and said feedback control loop providing control of saidvariable optical attenuator to reduce optical power transients andmaintain a substantially constant output power based on the outputoptical power measured by said optical power sensing element and areference value, wherein said reference value comprises a valueproportional to one of a desired output power level and a desiredoptical attenuation, and wherein said reference value is adjustable by auser.
 2. The apparatus according to claim 1, said variable opticalattenuator being biased to a non-zero value in order to compensate forboth positive and negative optical power transients.
 3. The apparatusaccording to claim 1, said variable optical attenuator being mid-rangebiased to a value substantially in the middle of said variable opticalattenuator's dynamic range in order to compensate for both positive andnegative optical power transients.
 4. The apparatus according to claim1, wherein said variable optical attenuator is a current injectionvariable optical attenuator.
 5. The apparatus according to claim 1, saidfeedback loop controller including: a feedback amplifier operativelyconnected to said optical power sensing element and inputting the signalindicative of output optical power of the output optical signal; a loopfilter operatively connected to said feedback amplifier and implementingfeedback control; and an output driver operatively connected to saidloop filter and to said variable optical attenuator, said output driverdriving said variable optical attenuator according to a control signalfrom said loop filter.
 6. The apparatus according to claim 5, whereinsaid feedback amplifier is a logarithmic amplifier.
 7. The apparatusaccording to claim 5, said loop filter applying a P, PI or PID feedbackcontrol scheme.
 8. An optical network comprising: a plurality of networkelements connected in a network configuration and carrying an opticalsignal having at least one channel; and an apparatus for suppressingoptical power transients according to claim 1, said apparatus forsuppressing optical transients being interposed between two of saidnetwork elements, wherein the network configuration is a linear, ring ormesh configuration.
 9. The optical network according to claim 8, saidapparatus for suppressing optical power transients being placed at alocation selected from a group consisting of before an amplifier networkelement, after an amplifier network element, before a receiver networkelement, between a transmitter and multiplexer, before amultiplexer/demultiplexer network element, after amultiplexer/demultiplexer network element, and at an express path of anadd/drop multiplexer network element.